The present invention relates to an antenna tracking device and more particularly an antenna tracking device capable of simultaneously tracking a plurality of stationary communication satellites spaced apart from each other by a relatively very short distance.
Due to the development of communication satellites, there has been realized a direct broadcasting satellite system in which a stationary satellite directly transmits broadcast waves to general viewers and listeners and the transmitted broadcast waves can be received by means of simple receivers. In the direct broadcast satellite system, in order that the viewers and listeners may install an antenna for receiving a plurality of broadcast programs, a plurality of frequencies (for example, 8 frequency bands at 110 degree of east longitude are assigned to the same stationary orbit position. Meanwhile, when one stationary satellite broadcasts a plurality of broadcast programs each with a high transmission power so that the viewers on the earth can receive them by means of simple receivers, there arises a problem that from the viewpoint of technology and cost it is difficult to fabricate a large-sized stationary satellite and launch it to a stationary orbit. As a result, a plurality of stationary satellites must be placed on the same orbit position or positioned in closely spaced apart relationship. In this case, a plurality of satellites transmit a plurality of beacon signals at different frequencies, respectively, and a ground control station receives them and controls the orbit positions, but their relative positions change due to variations in orbit inclination angle and in drift on the orbit. Since the antennas used by the general viewers are small in size and have a wide beam, the reception is not adversely affected by the positional variations. However, in the uplink for transmitting the program from the earth to a communication satellite, in general, a large-aperture, high-gain and narrow-beam antenna is used. As a result, there arises a problem that when the antenna is directed to one satellite, the gains in the directions to the remaining satellite are decreased. It is, however, economically very inadvantageous to install a plurality of uplink antennas on the earth so that each antenna is directed to each communication satellite.